1. Field of the Invention
The present invention generally relates to a Wireless Broadband (WiBro) system. More particularly, the present invention relates to a method for a method for determining the congestion state of the wireless network that causes a time delay and a stable algorithm for improving the quality of real-time video data transmission of video data in a WiBro system.
2. Description of the Related Art
Due to a wide bandwidth and a strict transmission delay constraint, there have been long-standing difficulties associated with efforts to commercialize video streaming service over a wireless network. Now that the recent increases in wireless access speed and the recent development of video compression technology have made the video stream service possible over the wireless network.
In particular, Korean Mobile Worldwide Interoperability for Microwave Access (WIMAX) or WiBro (IEEE 802.16e) has attracted a great deal of interest from mobile communication service providers as a future-generation wireless communication network due to a bandwidth of tens of Mbps and a mobility of tens of km/h. Yet, there are still difficulties in seamlessly reproducing video data between end-to-end entities. Radio channels change so much over the wireless network that the Bit Error Rate (BER) often changes a number of times per second. Also, the guarantee of high mobility likely results in frequent handovers, and delays occurs with increasingly frequency due to increasing network congestion.
Although errors generated at a PHYsical (PHY) layer end can be addressed by the use of an additional bandwidth, this problem may also be addressed by Forward Error Correction (FEC) and Automatic Repeat reQuest (ARQ) in terms of channel coding. In WiBro, Hybrid ARQ (HARQ) is adopted in view of end-to-end delay. To solve the shortcomings of WiBro that implements hard handover, studies are under way on improving the performance of a PHY Modulator-Demodulator (MODEM) and on many fast handover algorithms.
Typically, the quality of real-time video transmission is affected more by delay than by error-caused temporal loss, and handover over the wireless network. A delay constraint is related to a rate constraint in the real-time video transmission. If network congestion increases the time delay, a transmitter can experience buffer overflow, and a receiver may undergo buffer underflow. Consequently, seamless video transmission to the receiver is impeded and results in lost data. Therefore, when the state of the wireless network gets so poor as to impede seamless video transmission due to a time delay, it is necessary to reduce the video, output rate of the transmitter. If the state of the wireless network improves, the video output rate of the transmitter needs to be increased.
The above operation typically is governed by a parameter by which to evaluate the state of the wireless network, conventionally information about the PHY layer of the wireless network. For example, the main PHY layer parameters used in a WiBro network are Channel Quality Indicator (CQI), Adaptive Modulation and Coding (AMC) level, etc. The PHY layer parameters are feasible if an available bandwidth is sufficient. However, when the network is congested, the correlation between the transmitter and the receiver decreases considerably, thereby rendering the use of PHY layer parameters as being infeasible.
Accordingly, there exists a need in the art for a method of determining the congestion state of the wireless network that causes a time delay and a stable algorithm for improving the quality of real-time video data transmission.